[Identity] Reorder Dockerfile contents

Summary: Addressing a comment from @jimpo in a previous diff. We should install rustfmt immediately after setting the user.

Test Plan: Build the docker image

Reviewers: jimpo, atul

Reviewed By: atul

Subscribers: ashoat, palys-swm, Adrian, karol-bisztyga, yayabosh, jimpo

Differential Revision: https://phabricator.ashoat.com/D3818